1. Field of the Invention
This invention relates to fine carbon fibrous structures, particularly structures that are composed of fine carbon fibers wherein each of the fibers is composed of tubular laminates of fine carbon sheets and has a varied configuration. The fibrous structures are suitable as additives to be added to solid materials, such as resins, ceramics, metals, and etc., to improve the physical properties of the solid materials, such as electrical, mechanical, or thermal properties, and also as additives to be added to liquid materials, such as fuels, lubricant oils, and etc., to improve the physical properties of the liquid materials, such as thermal property.
2. Background Art
To date, carbon fibers have been used in various composite materials because of their good mechanical properties and high electrical conductivity. Recently, higher functionalities have come to be required for various materials. Additives which can improve physical properties, such as electrical, mechanical, or thermal properties, of a matrix comprised of solid materials, such as resins, ceramics, and metals, without damaging the characteristics of the matrix material have been sought after. Additionally, additives which can improve physical properties of liquids, such as fuels, oil, and lubricants have also been sought after. Hence, fine carbon fibers, such as carbon nano structures exemplified by the carbon nanotubes (hereinafter, referred to also as “CNT”.), have been attracting public attention in various fields.
The graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered rings whose structures can bring about specific electrical properties, as well as chemically, mechanically, and thermally stable properties. As long as such fine carbon fibers can retain such properties upon combining and dispersing into solid materials, including various resins, ceramics, metals, etc., or into liquid materials, including fuels, lubricant agents, etc., their usefulness as additives for improving material properties can be expected.
On the other hand, however, such fine carbon fibers unfortunately show an aggregate state even just after their synthesis. When these aggregates are used as-is, the fine carbon fibers would be poorly dispersed, and thus the product obtained would not benefit from the desirable properties of the nano structures. Accordingly, given a desired property such as electrical conductivity for a matrix such as a resin, it is necessary that the fine carbon fibers would be added in a large amount.
Japanese patent No. 2862578 discloses a resin composition comprising aggregates wherein each of the aggregate is composed of mutually entangled carbon fibrils having 3.5-70 nm in diameter, and wherein the aggregates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. It is noted that the numeric data such as the maximum diameter, diameter, and etc., for the carbon fibril aggregates are those measured prior to combining with a resin, as is clear from the descriptions in the examples and other parts of the patent literature. The related parts of Japanese patent No. 2862578 are incorporated herein by reference.
JP-2004-119386A discloses a composite material wherein a carbon fibrous material is added to the matrix. The carbon fibrous material is mainly comprised of aggregates, each of which is composed of carbon fibers having 50-5000 nm in diameter. The mutual contacting points among the carbon fibers are fixed with carbonized carbonaceous substance. Each of the aggregates has a size of 5 μm-500 μm. In this reference, the numeric data such as the size of aggregate, etc., are those measured prior to the combining into resin, too. The related parts of JP-2004-119386A are incorporated herein by reference.
Using carbon fiber aggregates such as those described above, it is expected that the dispersibility of carbon nano structures within a resin matrix will improve to a certain degree as compared with that of using bigger lumps of carbon fibers. Aggregates prepared by dispersing carbon fibrils under a certain shearing force, such as in a vibrating ball mill or the like, according to Japanese patent No. 2862578, however, have relatively high bulk densities. Thus, they do not fulfill the need for ideal additives that are capable of improving various characteristics, such as electrical conductivity, of a matrix effectively at minuscule dosages.
JP-2004-119386A discloses a carbon fibrous structure which is manufactured by heating carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression molding after synthesis of the carbon fibers, and wherein fixing of fibers at the contacting points is done by carbonization of organic residues primarily attached to the surface of the carbon fibers, or carbonization of an organic compound additionally added as a binder. Since fixing of carbon fibers is performed by such a heat treatment after synthesis of the carbon fibers, the affixing forces at the contacting points are weak and do not result in good electrical properties of the carbon fibrous structures. When these carbon fibrous structures are added to a matrix such as a resin, the carbon fibers fixed at the contacting points are easily detached from each other, and the carbon fibrous structures are no longer maintained in the matrix. Therefore, it is not possible to construct preferable conductive paths in a matrix such that good electrical properties may be conferred on the matrix by a small additive amount of the fibrous structures. Furthermore, when a binder is added to promote fixing and carbonization at the contacting points, fibers in the obtained fibrous structures would have large diameters and inferior surface characteristics because the added binder is attached to the whole surface area of the fibers rather than to a limited area on the contacting points.